1. Field of the Invention
The present invention relates to an ink ejecting device for forming images on a recording medium, such as paper, by ejecting ink from nozzles in accordance with print commands.
2. Description of the Related Art
Ink jet printers are a type of non-impact printer. They are based on the simplest operation principle and are also easily adapted for both tonal and color printing. Drop-on-demand type ink jet printers, which eject only ink required for printing, have good ejection efficiency and low running costs and so are becoming rapidly more popular. Conventional ink ejection units are described in U.S. Pat. Nos. 4,879,568, 4,887,100, 4,992,808, 5,003,679, and 5,028,936 and in Japanese Patent Application Publication (Kokai) No. SHO-63-247051, which corresponds at least partially to all of these U.S. patents.
Next, an explanation will be provided for an exemplary conventional shear mode type ejection unit that uses piezoelectric material for ejecting ink droplets. The ink ejection unit includes a plurality of ink chambers. Each ink chamber is defined by side walls made from piezoelectric material, and a nozzle plate formed with a nozzle through which ink droplets are ejected from the ink chamber. At the lengthwise end opposite the nozzle plate, the ink chamber is in fluid connection with a manifold through which ink is supplied to the ink chamber.
When an ink droplet is to be ejected from an ink chamber, a voltage is applied to the piezoelectric wells of that chamber, so that the walls of the chamber deform in a direction for increasing volume of the ink chamber and for reducing pressure in the ink chamber. The application of voltage is continued for a time T required for a pressure wave to propagate once from the nozzle plate to the manifold, that is, along the lengthwise direction of the ink chamber. During this time, ink is drawn from into the ink chamber from the manifold. The time T is calculated using the following formula:
T=L/a 
where L equals the length of the ink chamber; and
a equals speed of sound through the ink filling the ink chamber.
According to theories on pressure wave propagation, when the time T elapses after start of application of the voltage to the side walls, the pressure in the ink chamber inverts to a positive pressure. At this timing, the voltage applied to the piezoelectric side walls of the ink chamber is switched to zero volts, whereupon the piezoelectric side walls revert to their condition of before deformation, thereby applying pressure to the ink filling the ink chamber. This pressure applied to ink in the ink chamber by the side walls is added to the positive inverted pressure from the pressure wave. As a result, a fairly high pressure is generated near the nozzle of the ink chamber, so that an ink droplet is ejected from the nozzle.
It is well-known that pressure fluctuations, such as those described above, can be used for various purposes. For example, the pressure fluctuations can be used to eject a plurality of ink droplets in succession, in order to increase the surface area where ink clings to the recording medium. That is, when residual pressure wave vibration from a prior ink ejection inverts to a negative pressure, the piezoelectric walls can be applied with a voltage to deform them in the direction for increasing the volume of the ink chamber. When the residual pressure wave vibration inverts back to a positive pressure, then the piezoelectric walls are deformed in the direction for decreasing the volume of the ink chamber. As a result, a plurality of ink droplets are ejected in succession. The plurality of ink droplets either couple together in flight and impinge on the recording medium as a single layer droplet, or impinge on the recording medium separately as a plurality of ink droplets shifted slightly from each other, but in an overlapping condition. In other words, by changing the number of ink droplets ejected in succession, the surface area of ink clinging to the recording medium can be controlled.
Alternatively, the pressure fluctuations can be used to cancel out vibration in the ink in order to prevent undesired ink ejections. In this one, when the residual pressure wave vibration from a prior ejection inverts to a position pressure, the piezoelectric walls are deformed in the direction for increasing the volume in the ink chamber. Afterwards, when the vibration of the residual pressure wave inverts to a negative pressure, the actuator walls are deformed in the direction for decreasing the volume in the ink chamber. As a result, the residual pressure wave vibration is canceled out. The residual pressure wave vibration in the ink chamber undesirably pushes the ink meniscus out of the nozzle or pulls the meniscus into the ink chamber. However, when the residual pressure wave is canceled out in this manner, it has been proven experimentally that undesired ejection of ink from the nozzle can be prevented.
Tonal printing, wherein different tones are achieved by printing in different densities of ink, can be performed using this type of ink ejection unit by changing the number of ink droplets ejected onto the recording medium for each dot""s worth of print data. Conventionally, a ROM or other storage device stores separate date for different drive signal types. For example, the ROM or other memory medium stores data for a drive signal for ejecting a single ink droplet for each dot, a drive signal for ejecting two ink droplets for each dot, a drive signal for ejecting three ink droplets for each dot, and the like. Each time a print command is received from an external device, such as a computer, the data for the corresponding drive signal is retrieved from the ROM or other memory medium, and the drive signal is prepared accordingly.
To properly cancel out residual pressure wave vibration, the signal for canceling out the residual pressure wave vibration must be applied at a fixed timing after a prior ejection signal. Therefore, when a plurality of ink droplets are ejected in succession, the signal for canceling out is added after the plurality of ejection signals. For this reason, the ROM or other memory also stores signals for canceling out the residual pressure wave vibration, separately for each drive signal for ejecting a plurality of ink droplets.
Because data for a variety of different types of drive signal needs to be stored, conventional ink ejection units need to be provided with a memory medium having a relatively large capacity.
It is an objective of the present invention to overcome the above-described problems and to provide an inexpensive ink ejecting device that changes the number of ink droplets ejected in accordance with print data for each dot and that requires only a small capacity memory medium.
According to one aspect of the present invention, an ink ejecting device that prints single dots on a recording medium by ejecting one or more droplets, includes an ink chamber portion, an actuator that applies pressure to the ink in the ink chamber to perform an ink ejection to eject an ink droplet from the nozzle, and a drive unit.
The ink chamber portion is formed with an ink chamber and a nozzle. The ink chamber is filled with ink and the nozzle is in fluid communication with the ink chamber.
The actuator applies pressure to the ink in the ink chamber to perform an ink ejection to eject an ink droplet from the nozzle.
The drive unit applies a drive signal to the actuator to drive the actuator to perform ink ejections. The drive unit includes a generation unit and a correction unit. The generation unit prepares a reference drive signal for driving the actuator to perform a preset maximum number of ink ejections per dot. The correction unit produces a print drive signal for driving the actuator to perform a number of ink ejections required to print a particular single dot. The correction unit produces the print drive signal by removing unnecessary portions from the reference signal, according to the number of ink ejections required to print the particular single dot.
With this configuration, the generation unit of the drive unit prepares the reference drive signal for ejecting the maximum number of ink droplets for printing a single dot. The correction unit of the drive unit removes unnecessary portions from the reference drive signal in accordance with the number of ink droplets required to print the particular dot. The correction unit then applies the resultant signal to the actuator. For example, when the maximum number of ejections performed by the ink ejecting device is three ejections in succession, the generation unit always prepares a reference drive signal for ejecting three ink droplets for printing a single dot. In this case, when a print command is received for printing a particular dot by ejecting two ink droplets, the correction unit removes unnecessary portions from the reference drive signal to produce a print drive signal for performing two ink ejections for printing the particular dot. Then, the correction unit applies the resultant application drive signal to the actuator, whereupon the actuator is driven to eject ink filling the ink chamber from the nozzle.
The ink ejecting unit only needs to prepare the reference drive signal, remove unnecessary portions from the reference drive signal according to the number of ink ejections required to print the particular dot, and then apply the resultant signal to the actuator. Therefore, there is no need to store data for a variety of different drive signals. As a result, the memory capacity of the memory medium can be quite small so that the cost of the memory medium can be greatly reduced.
It is desirable that the generation unit generate the reference drive signal configured from a maximum number of ejection pulses in series, with adjacent ejection pulses separated by a predetermined time interval. In this case, each ejection pulse is for performing a single ink ejection to eject a single ink droplet and the maximum number of ejection pulses is equal to the preset maximum number of ink ejections per dot. Also, the correction unit produces the print drive signal by removing one or more ejection pulses from the maximum number of ejection pulses in the reference drive signal.
With this configuration, the correction unit produces an application drive signal by removing from the reference drive signal, an ejection pulse of the difference between the maximum number of ejections, and the number of ink ejections required to print the particular dot. For example, when the maximum number of ejections performed by the ink ejecting device is three ejections in succession, then in order to print a single dot using two ink ejections in succession, the correction unit removes a pulse for a single ink ejection as an unnecessary portion from the reference drive signal. The resultant signal is applied to the actuator. This configuration could be realized by an AND gate inputted with a reference drive signal and a correction signal, wherein the correction signal results in unneeded ejection pulses in the reference drive signal reverting to a low level. The output from the AND gate, that is, the logical sum of the reference drive signal and the correction signal, is then applied to the actuator.
An ink ejecting unit with this configuration can perform operations for removing unnecessary portions from the reference drive signal in order to more easily and accurately produce the application drive signal.
The present invention can be applied to a variety of different ink ejecting devices that use different types of actuators. For example, the actuator of the ink ejecting unit according to the present invention can be the type that heats ink to generate a vapor bubble in the ink filling the ink chamber, in order to eject ink droplets using force from the rapidly expanding vapor bubble.
Alternatively, the actuator can be a piezoelectric element or other element for converting electricity into mechanical force. As described previously, when a drive signal is applied to this type of element, the element deforms a wall or other portion of the ink chamber. The volume in the ink chamber changes as a result, thereby generating pressure wave vibration that applies ink ejection force to the ink in the ink chamber. With such a configuration, a plurality of ink drops can be effectively ejected in succession. Also, undesired ink ejection can be prevented. However, in this case, even if the number of ink droplets to be ejected changes, there is a need to cancel out the residual pressure wave vibration at a fixed timing after the last ejection.
Therefore, when the actuator ejects an ink droplet by changing volume of the ink chamber based on the print drive signal in order to generate pressure wave vibration in the ink chamber, it is desirable that the generation unit further generate a cancellation pulse in order to drive the actuator to substantially cancel out the pressure wave vibration in the ink chamber, and also that the generation unit generate the cancellation pulse after the maximum number of ejection pulses in the reference drive signal. In this case, it is desirable that the correction unit produce the print drive signal by removing one or more ejection pulses starting from a lead end of the reference drive signal.
With this configuration, even if the number of ejection pulses changes, a cancellation pulse for canceling out residual pressure wave vibration can always be generated at a fixed timing after the last ejection pulse. As a result, residual pressure wave vibration can be reliably canceled out. It should be noted that canceling as used in the present specification does not necessarily mean to completely eliminate residual pressure wave vibration, but also refers to a situation for suppressing residual pressure wave vibration to the extent where undesired ink droplet ejections are prevented.
According to another aspect of the present invention, the drive unit is configured differently, wherein the generation unit prepares an ejection pulse for driving the actuator once to perform a single ink ejection. The correction unit outputs the ejection pulse from the generation unit to the actuator in a dot print number equaling a number of ink ejections required to print a particular single dot. The correction unit outputs adjacent ejection pulses separated by a predetermined interval.
With this configuration, the generating unit of the drive unit prepares an ejection pulse for ejecting a single ink droplet. The correction unit of the drive unit outputs ejection pulses to the actuator in a number equaling a number of ink ejections required to print a particular single dot. Adjacent ejection pulses are separated by a predetermined interval. For example, the generating unit prepares only a single ejection pulse and the correction unit repeatedly outputs the ejection pulse a required number of times for printing the particular dot. Also, the generation unit prepares an ejection pulse for the maximum number of ejections used to print a single dot. The correction unit outputs the ejection pulse in a required number as determined by logic calculation of the ejection pulse and a signal that represents the number of ink ejections for the particular dot. With this configuration, there is no need to store data for a variety of different drive signals so that the memory medium needs only have a small capacity.
When the actuator is a type that ejects an ink droplet by changing volume of the ink chamber based on the print drive signal in order to generate pressure wave vibration in the ink chamber, it is desirable that the generation unit further generate a cancellation pulse to drive the actuator to substantially cancel out pressure wave vibration in the ink chamber generated when the actuator is driven by the print drive signal. In this case, it is desirable that the correction means output the print drive signal configured from the print number of the ejection pulses followed by the cancellation pulse after a predetermined interval.
With this configuration, the actuator changes the volume of the ink chamber and generates pressure wave vibration in the ink chamber accordingly. The actuator can efficiently eject a plurality of ink droplets in succession and also can prevent undesired ejections of ink droplets.
According to still another aspect of the present invention, configuration of the drive unit is different, wherein the generation unit prepares an ejection pulse for performing a single ink ejection, and a cancellation pulse for substantially canceling out the pressure wave vibration generated in the ink chamber by the ejection pulse. The correction unit outputs the ejection pulse from the generation unit to the actuator in a predetermined maximum number and then, after a predetermined interval, outputs the cancellation pulse to the actuator. The correction unit also outputs the ejection pulse from the generation unit to the actuator in dot print number equaling a number of ink ejections required to print a particular single dot and then, after a predetermined interval, outputs the cancellation pulse.
With this configuration, the generation unit of the drive unit prepares an ejection pulse for performing an ink ejection and also prepares a cancellation pulse for canceling out pressure wave vibration generated in the ink chamber by the ejection pulse.
The correction unit of the drive unit outputs the ejection pulse to the actuator in a number equaling a number of ink ejections required to print the particular single dot. In this case, pulses in the number required to print the particular single dot are disposed in the end side of the predetermined maximum number of pulses. The cancellation pulse is outputted after a predetermined interval after the final ejection pulse. For example, when the generation unit prepares a single ejection pulse and a single cancellation pulse, then the correction unit repeatedly outputs the ejection pulse in the required number. Also, the generating unit prepares an ejection pulse and a cancellation pulse, both based on a maximum number of ejections predetermined for print data for a single dot.